Nowadays, hard disk drives (hereinbelow, referred to as HDDs) rotate magnetic disks and drive head gimbal assemblies (hereinbelow, referred to as HGAs) at high speed in response to requests for huge capacity, high recording density, and high-speed accessing. These cause fluctuation of air (turbulence) to buffet the magnetic disks and HGAs. This turbulence buffeting can be a disturbance for positioning heads for data which are recorded with high-density on a magnetic disk. This is because the turbulence occurs at random and it is difficult to estimate its magnitude and cycle so that swift and accurate positioning control will be complex and difficult. Also the turbulence buffeting may cause a noise to impair the quietness of the device.
Another problem caused by influence of the air within the device due to the high-speed rotation other than the foregoing is increase of electric power consumption. When the magnetic disk is rotated at high-speed, the air around the disk is dragged and rotated together. On the other hand, the air apart from the magnetic disk remains still so that shearing force arises therebetween to become a load against the rotation of the disk. This is called as a windage loss, which becomes larger as the disk rotates at higher speed. In order to rotate the disk at high-speed against the windage loss, a motor will require a larger output and electric power.
Focusing on that the above-described turbulence and windage loss are proportional to the density of the gas within the device, there is an idea to reduce the turbulence and windage loss by enclosing low-density gas instead of air in a hermetically-sealed HDD. Hydrogen, helium, or the like is exemplified as the lower-density gas than air, but helium is optimum because it is effective, stable, and safe in considering actual use. HDDs with sealed helium gas can solve the above-described problems and realize swift and accurate positioning control, electric power saving, and satisfactory quietness.
However, molecules of helium are so small and a diffusion coefficient of helium is large. Therefore, there has been a problem that enclosures used for usual HDDs are poorly sealed so that helium gas leaks easily during normal use. In order to make it possible to hermetically seal low-density gas such as helium gas, for example, a technique disclosed in U.S. Patent Application Publication No. 2005/0068666 (“Patent Document 1”) has been suggested.
Specifically, since helium gas escapes easily as described above, welding or solder jointing is considered as appropriate for sealing an HDD. If an HDD is determined that it is defective at an inspection after assembled, parts in the HDD are replaced to repair the HDD. In order to perform this repair easily, it is desirable not to weld or solder joint until the inspection is completed after helium gas has been sealed.
The above-described Patent Document 1 discloses a method for sealing the HDD with doubled covers as one approach to solve this dilemma. This method seals the HDD, until the end of the inspection step, with a primary cover having a low helium-gas-permeable gasket which can be easily disassembled and replaced. Then, after passing the inspection, it attaches a secondary cover and seals the HDD by welding or soldering the second cover. Thus, utilizing a double cover structure enables to hermetically seal the HDD with helium gas enclosed therein and to disassemble and replace the components easily after the inspection.
On this occasion, in order to inhibit the helium gas from leaking from the finished HDD, the welded or soldered sealing section of the secondary cover should be inspected and confirmed that the helium gas is not leaking. However, if the above-described double cover structure is adopted, since a certain level of sealing property is secured by the primary cover, leakage may not be able to be detected at a detection step notwithstanding presence of a leak hole at the sealing section of the secondary cover.
Then, in order to assure the leak inspection on the secondary cover, a ventilation hole may be provided on the primary cover to flow out a certain quantity of helium gas therethrough. However, if merely the ventilation hole on the primary cover is provided, large amount of helium gas leaks at the time of fixing the secondary cover so that necessary helium gas may not be left in the HDD.
Besides, in the fixing step of the secondary cover by welding or soldering, dust and gas may be generated. Especially, generation of gas in welding will be a problem. If these dust and gas enter the interior space where a magnetic disk and the other components are placed, reliability in a head-disk interface will be reduced. Therefore, it is important to prevent the dust and gas from entering the interior space through the ventilation hole in joining.